First-Principles Molecular-Dynamics Simulations of Electrochemical Machining in Ultrapure Water

超纯水电解加工过程是利用超纯水中的水分子在催化剂作用下发生催化反应而解离产生OH离子，使加工表面与OH离子发生化学反应而实现的。该方法是一种最清洁和超精密的加工方法，可以避免电解液，加工过程后的清洗过程和废水对工作表面的污染。除了硅和铝以外，任何金属都可以在实际速度下加工，因为它们的表面都发生阳极氧化。在这项研究中，使用第一性原理分子动力学模拟的表面化学反应进行分析，以揭示在阳极和阴极表面的化学反应过程。模拟的基础上进行的Kohn-Sham局域密度泛函形式主义。采用平面波基组和范数守恒赝势。采用标准分子动力学方法对离子体系进行了优化，采用预处理共轭梯度法（CG）对电子自由度进行了猝灭。本文测定了Si（001）和Al（001）表面化学吸附H、H_2O和OH的最佳原子构型和电子分布。结果表明，H原子与氢封端的Si（001）表面或Al（001）表面上的H_2O分子反应生成OH分子，H原子和OH分子与表面原子的化学吸附破坏了背键，表面原子被刻蚀形成SiH_2（OH）_2或AlH_2OH分子。从这些模拟结果，我们推断，电解加工的Si和Al阴极表面在超纯水中是可能的。此外，加工实验中，样品作为阴极已被执行，它被证实，硅和铝都是可能的加工。

Electrochemical machining process in ultrapure water is indnced by chemical reaction between work surface and OH ions which are generated by dissociations of water molecules by catalytic reaction in ultrapure water using catalyst. This method is one of the most ultraclean and ultraprecision machining method which can avoid contaminations of work surfaces by electrolytes, cleaning processes after machining process and waste water. Any metals can be machined in practical velocity except Si and Al since the anodic oxidations occur on their surfaces. In this study, analysis of surface chemical reactions using first-principles molecular-dynamics simulations was performed in order to reveal the chemical reaction processes on the anode and cathode surfaces. The simulation were carried out on the basis of the Kohn-Sham local-density-functional formalism. A plane-wave basis set and a norm-conserving pseudopotential were used. The standard molecular-dynamics method was adopted for the optimization of the ionic system and the preconditioned conjugate-gradient (CG) method for the quenching procedure of the electronic degrees of freedom. We determined the optimized atomic configurations and electronic distributions for H, H_2O and OH chemisorbed Si(001) and Al(001) surfaces. It was confirmed that an H atom reacts with H_2O molecules on the hydrogen-terminated Si(001) surface or Al(001) surface to produce an OH molecule, and chemisorptions of H atoms and OH molecules to the surface atom breaks the back-bonds and the surface atom is etched with forming an SiH_2(OH)_2 or AlH_2OH molecule. From these simulation results, we deduced that the electrochemical machining of Si and Al cathode surface in ultrapure water is possible. Furthermore, machining experiments in which samples act as cathodes have been performed and it was confirmed that both Si and Al are possible to be machined.